Great efforts have been devoted to the development of advanced electrochemical battery cells to meet the growing demand of various consumer electronics, electrical vehicles and grid energy storage applications in terms of high energy density, high power performance, high capacity, long cycle life, low cost and excellent safety. In most cases, it is desirable for a battery to be miniaturized, light-weighted and rechargeable (thus reusable) to save space and material resources.
In an electrochemically active battery cell, a cathode and an anode are immersed in an electrolyte and electronically separated by a separator. The separator is typically made of porous polymer membrane materials such that metal ions released from the electrodes into the electrolyte can diffuse through the pores of the separator and migrate between the cathode and the anode during battery charge and discharge. The type of a battery cell is usually named from the metal ions that are transported between its cathode and anode electrodes. Various rechargeable secondary batteries, such as nickel cadmium battery, nickel-metal hydride battery, lead acid battery, lithium ion battery, and lithium ion polymer battery, etc., have been developed commercially over the years. To be used commercially, a rechargeable secondary battery is required to be of high energy density, high power density and safe. However, there is a trade-off between energy density and power density.
Lithium ion battery is a secondary battery which was developed in the early 1990s. As compared to other secondary batteries, it has the advantages of high energy density, long cycle life, no memory effect, low self-discharge rate and environmentally benign. Lithium ion battery rapidly gained acceptance and dominated the commercial secondary battery market. However, the cost for commercially manufacturing various lithium battery materials is considerably higher than other types of secondary batteries.
In a lithium ion battery, the electrolyte mainly consists of lithium salts (e.g., LiPF6, LiBF4 or LiClO4) in an organic solvent (e.g., ethylene carbonate, dimethyl carbonate, and diethyl carbonate) such that lithium ions can move freely therein. In general, aluminum foil (e.g., 15˜20 μm in thickness) and copper foil (e.g., 8˜15 μm in thickness) are used as the current collectors of the cathode electrode and the anode electrode, respectively. For the anode, micron-sized graphite (having a reversible capacity around 330 mAh/g) is often used as the active material coated on the anode current collector. Graphite materials are often prepared from solid-state processes, such as grinding and pyrolysis at extreme high temperature without oxygen (e.g., graphitization at around 3000° C.). As for the active cathode materials, various solid materials of different crystal structures and capacities have been developed over the years. Examples of good cathode active materials include nanometer- or micron-sized lithium transition metal oxide materials and lithium ion phosphate, etc.
Cathode active materials are the most expensive component in a lithium ion battery and, to a relatively large extent, determines the energy density, cycle life, manufacturing cost and safety of a lithium battery cell. When lithium battery was first commercialized, lithium cobalt oxide (LiCoO2) material is used as the cathode material and it still holds a significant market share in the cathode active material market. However, cobalt is toxic and expensive. Other lithium transition metal oxide materials, such as layered structured LiMeO2 (where the metal Me=Ni, Mn, Co, etc.; e.g., LiNi0.33Mn0.33Co0.33O2, with their reversible/practical capacity at around 140˜150 mAh/g), spinel structured LiMn2O4 (with reversible/practical capacity at around 110˜120 mAh/g), and olivine-type lithium metal phosphates (e.g., LiFePO4, with reversible/practical capacity at around 140˜150 mAh/g) have recently been developed as active cathode materials. When used as cathode materials, the spinel structured LiMn2O4 materials exhibit poor battery cycle life and the olivine-type LiFePO4 materials suffer from low energy density and poor low temperature performance. As for LiMeO2 materials, even though their electrochemical performance is better, prior manufacturing processes for LiMeO2 can obtain mostly agglomerates, such that the electrode density for most LiMeO2 materials is lower as compared to LiCoO2. In any case, prior processes for manufacturing materials for battery applications, especially cathode active materials, are too costly as most processes consumes too much time and energy, and still the qualities of prior materials are inconsistent and manufacturing yields are low.
Conventional material manufacturing processes such as solid-state reaction (e.g., mixing solid precursors and then calcination) and wet-chemistry processes (e.g., treating precursors in solution through co-precipitation, sol-gel, or hydrothermal reaction, etc., and then mixing and calcination) have notable challenges in generating nano- and micron-structured materials. It is difficult to consistently produce uniform solid materials (i.e., particles and powders) at desired particle sizes, morphology, crystal structures, particle shape, and even stoichiometry. Most conventional solid-state reactions require long calcination time (e.g., 4-20 hours) and additional annealing process for complete reaction, homogeneity, and grain growth. For example, spinel structured LiMn2O4 and olivine-type LiFePO4 materials manufactured by solid-state reactions require at least several hours of calcination, plus a separate post-heating annealing process (e.g., for 24 hours), and still showing poor quality consistency. One intrinsic problem with solid-state reaction is the presence of temperature and chemical (such as O2) gradients inside a calcination furnace, which limits the performance, consistency and overall quality of the final products.
On the other hand, wet chemistry processes performed at low temperature usually involve faster chemical reactions, but a separate high temperature calcination process and even additional annealing process are still required afterward. In addition, chemical additives, gelation agents, and surfactants required in a wet chemistry process will add to the material manufacturing cost (in buying additional chemicals and adjusting specific process sequence, rate, pH, and temperature) and may interfere with the final composition of the as-produced active materials (thus often requiring additional steps in removing unwanted chemicals or filtering products). Moreover, the sizes of the primary particles of the product powders produced by wet chemistry are very small, and tends to agglomerates into undesirable large sized secondary particles, thus affecting energy packing density. Also, the morphologies of the as-produced powder particles often exhibit undesirable amorphous aggregates, porous agglomerates, wires, rods, flakes, etc. Uniform particle sizes and shapes allowing for high packing density are desirable.
The synthesis of lithium cobalt oxide (LiCoO2) materials is relatively simple and includes mixing a lithium salt (e.g., lithium hydroxide (LiOH) or lithium carbonate (Li2CO3)) with cobalt oxide (Co3O4) of desired particle size and then calcination in a furnace at a very high temperature for a long time (e.g., 20 hours at 900° C.) to make sure that lithium metal is diffused into the crystal structure of cobalt oxide to form proper final product of layered crystal structured LiCoO2 powders. This approach does not work for LiMeO2 since transition metals like Ni, Mn, and Co does not diffuse well into each other to form uniformly mixed transition metal layers if directly mixing and reacting (solid-state calcination) their transition metal oxides or salts. Therefore, conventional LiMeO2 manufacturing processes requires buying or preparing transitional metal hydroxide precursor compounds (e.g., Me(OH)2, Me=Ni, Mn, Co, etc.) from a co-precipitation wet chemistry process prior to making final active cathode materials (e.g., lithium NiMnCo transitional metal oxide (LiMeO2)).
Since the water solubility of these Ni(OH)2, Co(OH)2, and Mn(OH)2 precursor compounds are different and they normally precipitate at different concentrations, the pH of a mixed solution of these precursor compounds has to be controlled and ammonia (NH3) or other additives has to be added slowly and in small aliquots to make sure nickel (Ni), manganese (Mn), and cobalt (Co) can co-precipitate together to form micron-sized nickel-manganese-cobalt hydroxide (NMC(OH)2) secondary particles. Such co-precipitated NMC(OH)2 secondary particles are often agglomerates of nanometer-sized primary particles. Therefore, the final lithium NMC transitional metal oxide (LiMeO2) made from NMC(OH)2 precursor compounds are also agglomerates. These agglomerates are prone to break under high pressure during electrode calendaring step and being coated onto a current collector foil. Thus, when these lithium NMC transitional metal oxide materials are used as cathode active materials, relatively low pressure has to be used in calendaring step, and further limiting the electrode density of a manufactured cathode.
In conventional manufacturing process for LiMeO2 active cathode materials, precursor compounds such as lithium hydroxide (LiOH) and transitional metal hydroxide (Me(OH)2 are mixed uniformly in solid-states and stored in thick Al2O3 crucibles. Then, the crucibles are placed in a heated furnace with 5-10° C./min temperature ramp up speed until reaching 900° to 950° C. and calcinated for 10 to 20 hours. Since the precursor compounds are heated under high temperature for a long time, the neighboring particles are sintered together, and therefore, a pulverization step is often required after calcination. Thus, particles of unwanted sizes have to be screened out after pulverization, further lowering down the overall yield. The high temperature and long reaction time also lead to vaporization of lithium metals, and typically requiring as great as 10% extra amount of lithium precursor compound being added during calcination to make sure the final product has the correct lithium/transition metal ratio. Overall, the process time for such a multi-step batch manufacturing process will take up to a week so it is very labor intensive and energy consuming. Batch process also increases the chance of introducing impurity with poor run-to-run quality consistency and low overall yield.
Thus, there is a need for an improved process and system to manufacture high quality, structured active materials for a battery cell.